Sweat Ceiling 2011

Readers, radio show hosts, and even meteorologists are questioning whether this is an unusual heat wave. I keep hearing this is just typical July heat. No it’s not. Let me explain...

Not to pick on radio commentator, Rush Limbaugh, but Wednesday he said, “We have this – there’s a heat dome over half the country. Midwest is moving East. And it happens every summer. Every summer.”

Referring specifically to Washington, D.C., he said: “It’s going to top out at 102 103. It does this every year.”

As Ian described yesterday, it’s true D.C. averages about one 100-degree day per year. But the combination of heat and humidity during this heat wave is unlike anything we’ve seen in 10-15 years. Prior to today, the last time Washington, D.C. had a 110 heat index was in August, 2002. And the last time it experienced a 115 heat index - possible tomorrow - was in July 1995 - more than 15 years ago.

Animation of the progression of dew points - a measure of humidity - from July 18-24. Source: NOAA

It’s the humidity the sets this heat wave apart - not just in D.C., but over large parts of the country and for a long duration.

Consider:

* Minneapolis not only set a new all-time record dew point (a measure of humidity) - of 82, but also recorded a dew point of at least 80 for three consecutive days for the first time ever. In addition, its heat index of 119 Tuesday tied for its highest on record.

* Heat indices exceeded 110 both Monday and Tuesday (and 120-130+ degrees in isolated locations) from Texas to the Canadian border in the central U.S..

* The heat index is forecast to exceed 100 over much of the eastern two-thirds of the nation both Friday and Saturday

* 148 million people in 32 states and Washington, D.C. are currently under heat advisories or warnings today. On Wednesday, 141 million people were under advisories/warnings whereas 78 million were Tuesday. The heat wave will continue impacting millions of American through at least Saturday.

* In the last week, 1518 record high minimum temperatures have been set or tied (compared to just 171 record low minimum temperatures). Record high low temperatures are a good indicator of the humidity levels, because humid air cools less readily than dry air.

In the drought stricken regions of Texas and Oklahoma - where it hasn’t been humid due to low soil moisture - the heat has been relentless:

* On Wednesday Amarillo, Texas tied a record of 103 and tied its record for most 100-degree days (26) with a large chunk of summer left to go

* Wichita Falls, Texas has 56 days of 100+ temperatures in 2011 and 29 consecutive days reaching at least 100

* Dallas’ temperature has reached 100 for 19 consecutive days, the sixth most on record

To conclude: where it’s been dry, the day time heat has been unusual; elsewhere, the night time heat and humidity has been unusual. It will take more time and analysis to objectively rank where this heat wave stacks up in the record books, but it’s clear that it’s far from the ordinary.

Post script: To support his argument this heat wave is nothing special, WeatherBell meteorologist Joe Bastarditweeted that he believed heat indices in the Northern Plains were probably much higher in the 1930s. He also noted summer record highs hadn’t been set in Des Moines since the 1950s (and that most occurred in the 1930s). But what he’s not considering is that there was record drought in the Northern Plains in the 1930s as nicely illustrated in a recent blog post by the Weather Channel’s Stu Ostro. Because it was so dry there in the 30s, it’s no surprise all of those record highs were set. But I’m entirely unconvinced those dry conditions would’ve also supported record heat indices.

Jason is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science, and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.